Learning Objectives
The Origin and Evolutionary
History of Life
1. What conditions do geologists think
existed on early Earth?
2. The prebiotic soup hypothesis and iron–
sulfur world hypothesis
• Hypotheses for the origin of cells?
• Evolution of photosynthetic autotrophs
and their affect?
Chapter 21
Early Earth
The Origin of Life
• Biologists generally agree:
1. life originated from nonliving matter
2. by chemical evolution
•How did life begin?
•Earth is ~ 4.6 billion years old
•CO2,H2O, CO, H2, N2, (& ammonia, hydrogen
sulfide, methane)
•NO O2
KEY CONCEPTS
• There is no direct fossil evidence of the
origin of life
• Biochemical experiments have
demonstrated molecules that are found
in all living organisms, may have
formed
• Origin of life is difficult to test experimentally
– hypotheses about chemical evolution can be
tested
4 Requirements for needed for
Chemical Evolution of life
1. Absence of oxygen!!!
– oxygen would oxidize organic molecules
(steal their electrons)
- Atoms make molecules
- Organic molecules have Carbon atoms
- SO! O2 would have broken down organic
molecules
- O2 would not have been available
(bonded to other molecules
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4 Requirements for needed for
Chemical Evolution of life
4 Requirements for
Chemical Evolution
2. Energy - to form organic molecules
3. Chemical building blocks to build organic molecules
– water, minerals, gases in atmosphere
– to form organic molecules
4. Lots of time - for molecules to accumulate and
react
- Earth = 4.6 bya; Life = 3.5 bya
Electrodes
Miller and Urey’s
How did Organic Molecules occur?
Experiment
NH3
CH4
• 2 Hypothesis
• Prebiotic soup hypothesis
– organic molecules formed near Earth’s
surface “sea of organic soup” or on rock
or clay surfaces
• Iron–sulfur world hypothesis
– organic molecules formed at hydrothermal
vents in deep ocean floor
H2
H2O
To
vacuum
Spark chamber
Condenser
Boiling
chamber
Heat source
Organic molecules
collect in the trap
Fig. 21-2, p. 449
Iron–sulfur world hypothesis
• Hydrothermal vents produce precursers of
biological molecules
• Chemicals support community of
microorganisms
• Lab experiments
- produce organic compounds
- Ammonia is produced (protein &
nucleic acids precurser)
The Origin of Cells
Protobionts (macromolecule assemblages)
– formed from macromolecules – spontaneously
– Grow
– Can divide
– Internal environment differs from external
environment
– Metabolism (chemical/energy changes)
Cells -arose from protobionts?????
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Protobiont: e.g., Microspheres
•Water + protein creates
•Excitable – electrical charge across their surface
•Can obsorb materials from environment
•“Precells??”
Molecular reproduction was important
step in origin of cells
• DNA → RNA → protein
• DNA and RNA can replicate if right molecules
are present
• Important with respect to cell division
RNA
The First Cells
RNA world: prebiotic earth gave
rise to self-replicating RNA
Stromatolites
• 2 types of cells
- Prokaryotes (no nucleus, no organelles)
- Eukaryotes (nucleus & organelles)
• When did first cells appear on Earth?
• Who was first Prokaryotes or Eukaryotes?
- Microfossils 3.5 MYA or rock squiggles?
- Evidence suggests 1st cells were
Prokaryotes
• Prokaryotic heterotrophs
– “consumed” organic molecules from
environment
– probably anaerobes (No O2 then)
• Autotrophs - evolved later
– produced organic molecules by
photosynthesis
- algae
cynobacteria (3.1-3.5 bya)
- eventually put O2 in atmosphere
- Created Ozone layer
Photosynthesis
• Generated oxygen in atmosphere
– 2.3 bya lots of oxygen
– changed early life
– permitted evolution of aerobes
• Aerobes
– use oxygen for efficient cellular respiration
– Stabilized O2 and CO2 levels
– Carbon cycling
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Photosynthesis:
Serial Endosymbiosis Hypothesis
Also provided oxygen
that created Ozone
layer
• Eukaryotic cells descended from prokaryotic
cells
UV light breaks O2
bonds and forms O3
• Certain eukaryotic organelles (mitochondria,
chloroplasts) evolved from prokaryotic
endosymbionts
– Endosymbiont: organisms that live
symbiotically within another organism
Protects life from UV
light
mya
ORIGINAL
PROKARYOTIC
HOST CELL
Multiple invaginations of
the plasma membrane
Aerobic bacteria
become mitochondria
Photosynthetic
bacteria...
DNA
Aerobic bacteria
Endoplasmic reticulum
and nuclear envelope
form from the plasma
membrane invaginations
1st amphibians
1st terr. plants
... become
chloroplasts
EUKARYOTIC
CELLS
1st chordates
EUKARYOTIC
CELLS
The Paleozoic Era (1)
Cambrian Radiation
• Began about 542 mya
– lasted about 291 million years
• Many plants and animals appeared
– all major plants (except flowering plants)
– all animal phyla
– reptiles
– fishes and amphibians flourished
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The Paleozoic Era (2)
• Greatest mass extinction of all time
– at end of Paleozoic era (251 mya)
– > 90% of marine species extinct
– 70% of land-dwelling vertebrate genera
– many plant species
The Mesozoic Era
• Began about 251 mya
– lasted about 185 million years
• Dinosaurs dominated
– reptiles diversified
– insects flourished
– flowering plants appeared
– birds appeared
– early mammals appeared
Cenozoic Era
• From 66 mya to present
– flowering plants, birds, insects, mammals
diversified greatly
• Late Miocene and Early Pliocene epochs
– human ancestors appeared in Africa
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